(1) Field of the Invention
The present invention relates to a thin-film semiconductor epitaxial substrate suitable for producing a hetero-junction bipolar transistor (HBT) and a process for producing the thin-film semiconductor epitaxial substrate.
(2) Description of the Related Art
As a semiconductor element used in a region of frequencies higher than that of microwave, a high-performance hetero-junction bipolar transistor (HBT) is produced by using a recent excellent crystal growth technique such as MOCVD. HBT is composed as follows: a material having a larger band gap than a base layer is used as an emitter layer to make emitter-base junction as hetero junction, and an energy barrier at the time when holes flow into the emitter layer from the base layer is heightened as compared with a homo-junction bipolar transistor so that holes can be prevented from flowing into the emitter, whereby the emitter injection efficiency is improved.
The current amplification mechanism of conventional HBT composed as described above is fundamentally the same as that of a conventional homo-junction bipolar transistor. Accordingly, the probability of the recombination of electrons and holes in the base region should be lessened in order to improve the current amplification factor. Therefore, the following structure, for example, has been well known: AlGaAs with a compositional gradient is used in the emitter layer and base layer so that an electric field for accelerating electrons is produced, and the flow of electrons in the base region is accelerated by the synergistic effect of a conventional type of flow due to diffusion caused by the concentration gradient of electrons and a flow due to the transit of electrons caused by the electric field produced in AlGaAs crystals, to reduce the transit time of electrons in the base region, whereby the probability of the recombination of electrons and holes in the base region is lessened.
In addition, an attempt has been made to improve the crystallinity of the emitter layer and the base layer or the boundary surface between them in order to improve the current amplification factor of HBT. The relationship between the crystallinity of a collector layer or a sub-collector layer and the current amplification factor, however, has not sufficiently been investigated.
However, it has recently been found that the carrier concentration in a sub-collector layer, i.e. a thin layer with a high electric conductivity formed between a collector layer and a collector electrode in order to reduce the contact resistance between them, affects the current amplification factor remarkably. In designing a semiconductor device, it is desirable that the carrier concentration in the sub-collector layer can be freely set. However, when the carrier concentration affects the current amplification factor remarkably, the degree of freedom of the design of the device is severely limited, so that the design of a device having desirable characteristics is complicated.